The M33 Metallicity Project: Resolving the Abundance Gradient Discrepancies in M33

TL;DR

This study provides a precise measurement of M33's metallicity gradient using direct oxygen abundance data from 61 HII regions, revealing a shallower gradient and significant intrinsic scatter, with implications for neon abundance measurements.

Contribution

It offers the first extensive direct oxygen abundance measurements in M33, refining the metallicity gradient and highlighting the importance of ionization corrections for neon.

Findings

The metallicity gradient in M33 is shallower than previously thought.

There is substantial intrinsic scatter in metallicity at fixed radii.

Neon ionization state does not follow oxygen, affecting abundance measurements.

Abstract

We present a new determination of the metallicity gradient in M33, based on Keck/LRIS measurements of oxygen abundances using the temperature-sensitive emission line [OIII] 4363 A in 61 HII regions. These data approximately triple the sample of direct oxygen abundances in M33. We find a central abundance of 12 + log(O/H) = 8.36+/-0.04 and a slope of -0.027+/-0.012 dex/kpc, in agreement with infrared measurements of the neon abundance gradient but much shallower than most previous oxygen gradient measurements. There is substantial intrinsic scatter of 0.11 dex in the metallicity at any given radius in M33, which imposes a fundamental limit on the accuracy of gradient measurements that rely on small samples of objects. We also show that the ionization state of neon does not follow the ionization state of oxygen as is commonly assumed, suggesting that neon abundance measurements from optical emission lines require careful treatment of the ionization corrections.